1. Field of the Invention
The present invention relates to an elastic blade for controlling the amount of feed of a developing agent (hereinafter referred to as a "developer") for developing and visualizing an electrostatic latent image formed on an image-holding member, and also to a development device employing the elastic blade.
2. Related Background Art
Known development devices, as shown in FIG. 7, comprise a developer-bearing member 3 (hereinafter referred to occasionally as a "development sleeve") which is attached to a development vessel 2 at a small distance to an electrophotographic photosensitive member 1, an elastic blade 14 for controlling the amount of the feed of a developer, an elastic roller 5, and a one-component developer 6 (hereinafter referred to also as a "toner"). The elastic blade is brought into contact with the development sleeve to control the thickness of the toner layer delivered to the development section. A thin toner layer is formed on a development sleeve by allowing the toner to pass through the contacting portion between the elastic blade and the development sleeve, and simultaneously given electrification (triboelectricity) for developing latent images by the friction at the contacting portion.
Such an elastic blade includes ones constituted of a rubber plate, a metal thin plate, a thin plastic plate, or a laminate thereof.
For a positive type toner, the elastic blade is used which is formed by laminating an electric charge-imparting layer (hereinafter referred to as an "electrifying layer"), such as a charge-controlled silicone rubber, onto a thin plate, such as a metal plate, as a supporting layer.
For a negative type toner containing magnetite, a urethane rubber sheet is used which has been subjected to charge control treatment.
On the other hand, a low-temperature melting toner (sharp-melting toner) which is used in view of energy saving involves a problem that the toner tends to be fusion-bonded to the development blade to cause defective image formation. This problem can be solved by decreasing the contact pressure between the development blade and the development sleeve. At the lower contact pressure, however, the development blade should have a surface layer having higher triboelectrification ability at lower contact pressure. The conventional material such as urethane rubber used therefor was found to be insufficient in triboelectrification ability, disadvantageously.
The non-magnetic toner, which has come to be used for color image formation, is required to be more highly electrified and to be applied onto the development sleeve because of the non-magnetic properties of the toner itself. Since the urethane rubber as the surface layer of the development blade is not sufficient in triboelectrification ability as mentioned above, a polyamide having high electrification ability is used as the surface layer.
However, in the case where the polyamide is used as the surface layer material of the development blade, an ordinary non-magnetic toner is electrified excessively under low humidity conditions. The excessive electrification (charge-up) prevents the toner from being attracted from the development sleeve to the photosensitive drum, thereby resulting in defective image formation.
Further, for higher image quality and full color image formation by electrophotography, a finer particle size of the toner and uniform contact pressure onto the development sleeve are required. However, in conventional elastic blades, there is the limitation of uniformity of the press-contact with the development sleeve in its axis direction, and therefore, uniformity of the electric charge and thickness of the applied toner is insufficient, resulting in image defects such as image irregularity and streaks.